     var code="";
	/**绘制验证码图片**/
    function drawPic() {
    	code="";
        var canvas = document.getElementById("vcode_img");
        var width = canvas.width;
        var height = canvas.height;
        //获取该canvas的2D绘图环境 
        var ctx = canvas.getContext('2d'); 
        ctx.textBaseline ='bottom';
        /**绘制背景色**/
        ctx.fillStyle = randomColor(180,240);
        //颜色若太深可能导致看不清
        ctx.fillRect(0,0,width,height);
        /**绘制文字**/
        var str ='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890';
　　　　　//生成四个验证码
        for(var i=1;i<=4;i++) {
            var txt = str[randomNum(0,str.length)];
            code=code+txt;
            ctx.fillStyle = randomColor(50,160);
            //随机生成字体颜色
            ctx.font = randomNum(15,40) +'px SimHei';
            //随机生成字体大小
            var x =10 +i *25;
            var y = randomNum(25,35);
            var deg = randomNum(-45,45);
            //修改坐标原点和旋转角度
            ctx.translate(x, y); 
            ctx.rotate(deg * Math.PI /180); 
            ctx.fillText(txt,0,0);
            //恢复坐标原点和旋转角度
            ctx.rotate(-deg * Math.PI /180);
            ctx.translate(-x, -y);
        }
        
      /**绘制干扰线**/     
         for(var i=0;i<3;i++) {
         ctx.strokeStyle = randomColor(40, 180);
          ctx.beginPath();
          ctx.moveTo(randomNum(0,width/2), randomNum(0,height/2));
          ctx.lineTo(randomNum(0,width/2), randomNum(0,height));
          ctx.stroke();
      }
     /**绘制干扰点**/
     for(var i=0;i <50;i++) {
          ctx.fillStyle = randomColor(255);
          ctx.beginPath();
          ctx.arc(randomNum(0, width), randomNum(0, height),1,0,2* Math.PI);
          ctx.fill();
      }
		return code;
  }
	
	    /**生成一个随机数**/
	    function randomNum(min, max) {
	        return Math.floor(Math.random() * (max - min) + min);
	    }
	    /**生成一个随机色**/
	    function randomColor(min, max) {
	        var r = randomNum(min, max);
	        var g = randomNum(min, max);
	        var b= randomNum(min, max);
	        return "rgb(" + r + "," + g + "," + b + ")";
	    }
	    